A substrate processing apparatus and method which can prevent accidents in advance so as to ensure safety against interruption of operations of the apparatus and leakage of processing substances. In a substrate processing method in which wafers W are processed are processed by feeding an ozone gas 5 to the wafers W loaded in a processing vessel 2 while an interior atmosphere in the processing vessel is being exhausted to be passed through an ozone killer 10, the ozone gas 5 is fed under the conditions that the processing vessel 2 is tightly closed, and the ozone killer in its normal state. When the processing is interrupted, an interior atmosphere in the processing vessel 2 is forcedly exhausted. When the gas leaks, the interior atmosphere and a peripheral atmosphere of the processing vessel 2 are forcedly exhausted while the feed of the ozone gas 5 is paused. The processing gas is fed under a condition that the processing vessel is tightly closed, and the post-treatment of the exhausted interior atmosphere is normally carried out.